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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Bachelor Thesis

Correlation and Interaction of Arsenic und Fluorine in Argentinian groundwaters

Benedikt Werner (11/2012-09/2013)

Support: Britta Planer-Friedrich

In parts of Argentina elevated arsenic levels were found next to increased fluoride content in the groundwater. An interaction of these two contaminants with the formation of new hexafluoroarsenate complexes (AsF6-) is known from synthetic chemistry. AsF6- has also been found from anthropogenic source in natural waters. This complex behaves toxicologically different to other As species and could provide an explanation for the observed phenomenon in Argentina of lower cancer rates compared to other countries with similar problems. This study investigated whether hexafluoroarsenate exists in a geogenic way. For this purpose, first an analytical method for the detection of AsF6- was established (eluent: 20 mM perchlorate in 2 mM NaOH). Subsequently, stability and formation tests of AsF6- took place in the laboratory. In these first tests Hexafluoroarsenate could neither be formed nor destabilized. In Argentina, a screening ensued, in which Fe, NO3-, PO43-, pH, temperature and conductivity were measured on site, arsenic species (AsO43-, AsO33-, AsF6-) and the total contents of As, V, Mo, Sb, Se, U, and Cr were determined by (IC-)ICP-MS. However hexafluoroarsenate could not be found in the samples. In view of possible correlations F-, Cl -, NO2-, Br-, NO3-, PO43- and SO42- were analyzed by IC. In the overall data set arsenic correlated with vanadium, fluoride showed no correlations. The maximum concentrations of As (923 µg L-1) and F (12.5 mg L-1) were found in aquifers of the upper 20 meters. In a clustering of three regions, Robles region revealed positive correlations between arsenic, fluoride (R ² = 0.61, ρ = 0.79), pH, Mo, V, U, and Br-, while F- and Ca2+/Mg2+ were correlated negatively. For samples from Termas de Rio Hondo, as well as Cordoba and Mendoza these correlations did not appear. With this work, however, it was possible to demonstrate that hexafluoroarsenate most likely did not exist in the argentinian ground water, so it could not exert any influence on the arsenic toxicity.

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